1. Field of the Invention
This invention relates to a method and an apparatus for electron beam welding. More specifically, it relates to a method for electron beam welding which comprises accelerating electrons by the application of a high voltage to produce an electron beam, converging the electron beam by means of a converging lens generally known as a magnetic lens, irradiating the converged electron beam onto materials to be welded thereby to impinge the electrons against the materials, and melting the materials by the heat of impact; and to an apparatus for effecting this method.
2. Description of the Prior Art
Electron beam welding is a technique for joining materials by utilizing a high kinetic energy of electron beam, and has the advantage of being able to afford a weld bead having a large penetration depth and a very narrow melting width. As is well known to those skilled in the art, however, electron beam welding has special problems. When electron beam welding is to be performed by melting the joining materials partly in their thickness direction (to be referred to as the "partial penetration welding procedure"), spikes, cold shuts and pores tend to form on the penetrating end portion of the molten area of the joining materials. In the case of melting the joining materials along their entire thickness (to be referred to as the "full penetration welding procedure"), the weld bead tends to become uneven on the back surface of the joining materials.
In an attempt to solve the aforesaid problems associated with electron beam welding by the partial penetration welding procedure, it has been suggested to reduce the power density of electron beam by increasing the diameter of the converged electron beam, or by continuously oscillating the electron beam sideways. Techniques based on these suggestions are in practical use. However, these techniques naturally lead to a decrease in weld penetration depth and an increase in bead width, and offset the aforesaid advantage of electron beam welding. In the case of welding by the full penetration welding procedure, on the other hand, it has been suggested to solve the aforesaid problem by increasing the electric current value of electron beam to more than that required for full penetration welding to cause the central portion of the electron beam which has a high power density to pass completely through the joining materials so as not to use the central portion of the electron beam for the melting of the joining materials, and melting the joining materials by the surface portion of the electron beam which has a low power density. Such a technique is already in use, but has the disadvantage that power consumption is exceedingly high.